If you're struggling to understand how convergent and
discriminant validity work together to help establish construct validity,
you are not alone! The vast majority of people who try to learn about
construct vality initially have trouble getting a handle on the way convergent
and discriminant validity work.

In an effort to help you pin down the meanings of convergent
and discriminant validity, I'd like to share a paragraph that comes out
of a book authored by Anne Anastasi, a highly respected individual who
made enormous contributions to the field of testing many years ago. Her
discussion of convergent and discriminant validity was brief and clear,
and it remains instructive to this day.

Here's Anastasi's paragraph:

"In a thoughtful analysis of construct validation,
D. T. Campbell (1960) pointed out that, in order to demonstrate construct
validity, we must show not only that a test correlates highly with other
variables with which it should theoretically correlate, but also that
it does not correlate with variables from which it should differ. In an
earlier article, Campbell and Fisk (1959) described the former process
as convergent validation and the latter as discriminant validation. Correlation
of a quantitative reasoning test with subsequent grades in a math course
would be an example of convergent validation. For the same test, discriminant
validity would be illustrated by a low correlation with scores on a reading
comprehension test, since reading ability is an irrelevant variable in
a test designed to measure quantitative reasoning."

OK. That's the paragraph I wanted to share. I hope you
found it as clear and as instructive as I did. I also hope it drove home
the point that discriminant validity is demonstrated by low (near zero)
correlations, not high negative correlations.

Sky Huck

P.S. If you haven't done so already, check out the 2nd
of Chapter 4's "on-line resources." Via a link, you'll be taken
to a page of another Website contains a more lengthly (and more colorful)
discussion of convergent and discriminant validity.